The present invention relates to a method of calibrating scanners and an arrangement for producing defined scattered light amplitudes, particularly for measuring particles and surface finishes during the inspection of substrates, for example wafers.
Devices which are based on the measurement of the elastic scattered light are used, inter alia, for the measurement of particles in air or on surfaces or for the inspection of surfaces (U.S. Pat. No. 4,314,763).
In such devices, the area to be observed is illuminated by a light source of high intensity and the scattered light is supplied to a photosensor--for example via a dark-field optical system. The photosensor (photodiode, photomultiplier) generates an electrical signal which is proportional to the amplitude of the scattered light and is fed to a following amplifier. On the assumption that the scattering capacity of a defect is related to its size, conclusions about the extent of the defect can be drawn from the scattered light amplitude.
This method of measuring has the disadvantage, however, that fluctuations in intensity of the light source, variations in the optical system (for example dark-field optical system or light source adjustment) and electronic drift in the photodetector or in the amplifier influence the result of the measurement at the output of the amplifier.
For this reason, attempts are made to compensate for such variations by means of various correction methods.
One such correction method consists in the regular measurement of the intensity of the light source. Fluctuations in the intensity can then be compensated for, as by varying the sensitivity of the photodetector or the amplifier. This method of correction has the disadvantage, however, that variations in the optical system, in the photodetector or in the amplifier, are not detected and are not taken into consideration.
Methods which include the whole of the light source, optical system, photodetector and amplifier for the calibration and measuring respectively are therefore better suited as correction methods.
Such methods used for this purpose a medium of known scattering capacity (reference medium) (as described for example in U.S. Pat. No. 4,512,659). Usually, in this case, the sensitivity of the photodetector and of the amplifier is altered with respect to the known medium until the desired value appears at the output of the amplifier. Thus with such a method, the accuracy of the calibration depends exclusively on the scattering capacity of the reference medium.
If very sensitive scattered-light devices are to be calibrated by this measuring method, however, the requirements regarding the reference medium have to be set particularly high, since a reference medium which produces very low scattered light amplitudes is extremely susceptible to variations (see in particular as described in U.S. Pat. No. 4,512,659).
If the reference medium is, for example, a surface with a very low scattering capacity, particles, air humidity, oxidation etc alter its scattering capacity drastically. In addition, the cleaning of such sensitive surfaces is extremely expensive and has to be carried out step-by-step in special cleaning processes. Contamination by particles has a particularly disadvantageous effect if the illuminated field is small. In this case, a single particle can alter the scattering capacity of the surface by some orders of magnitude.
Finally, the ability to produce surfaces thus prepared with an appropriately small scattering capacity limits the possible use of this method.